@ mba-man,
Assuming that the batterys are not defective or damaged, there is no difference in safety between sizes; the only differences will be the battery size, weight, and the length of runtime with your computer.

I’ve purchased cells and protection curcuts from batteryspace before, and their tech support people are knowledgeable, friendly, and quick to respond, even though I only make small just-for-my-own-use orders; I do recommend them.

Batteryspace offers a good variety of protection circuts, or you can search Ebay or Amazon for “battery protection circuit” for more choices; keep in mind that there are many different configurations, and be sure to get one that suits the number of cells in your pack.—This thread discusses 4S (four cells in Series) and 5S (five cells in Series) battery packs, and while some protection circuts will protect either, a circut designed to protect 2 to 4 cells will not protect a 5 cell pack.

The protection circuts designed for model aircraft are intended mainly to cut power to the model’s motor, while leaving the control servos powered, so that the aircraft can be landed instead of crashing. These can easily be adapted to cut power output without giving an audible warning, but that may be beyond what many here are capable of. That being said, a loud alarm won’t do any harm beyond annoying people, and it will protect the battery pack.

I've got one more question about this. Are there any chargers available that can prevent overcharging the LiPo battery? I'm just thinking that I would not know exactly when to unplug if I'm not near the battery after several hours while it's charging up.

mba-man, go back through the thread and find a mention of one that you feel confident with. If your battery has the balance connector, get a balanced charger, and nothing will do you better than that. Believe me, those model flyers on HobbyKing's forums are downright serious about their batteries.

So, I've finally got my HB kit !!! Here it is :Image
I've used a car charger and some adapters so I can continue to use it and have also the battery pack. Cables are long enough to allow me to charge my MacBook Air from my backpack

This was really a great idea ! Total cost : $ 60

Wish I could make one of these.. I am not really an electronics guy (familiar with li-on batteries and have some chargers) but not sure about some of the cabling. Can these cables be bought ready to use?

mayhem28, badman89, hlkc, and anyone else leery of cutting wires and soldering, there are already people selling ready-to-use external batteries for MacBooks, but no one could possibly sell one for anything near $60; it costs that much just to get the MagSafe charging cable.

mikegyver.com does sell magsafe cables—and they will also put an adapter plug on your cable for $20—but you’ll still need to get the battery wired to it. If the battery that you use doesn’t have a protection circut, you’ll need to get one of those in there, too.

The bottom line, I think, is that if you can’t do it yourself, then you can’t save any money; just go ahead and buy a complete product. If you can’t afford or justify spending over $200 for a solution, then you might give some thought to spending $20 or $30 to learn to solder, and I don’t mean to be a smart aleck—its a useful skill.

Well hobbyking was extremely stubborn about my battery. It took me six emails for them to answer my original question of if they will send me my defective battery back or not. Finally they sent it back. So 3 months later and im stuck with my broken battery again.

Now I have a question for all you battery gurus. The far right cell has expanded and is defective. Can I just take that cell out and use the battery as a 4s battery??
Cheers

Well hobbyking was extremely stubborn about my battery. It took me six emails for them to answer my original question of if they will send me my defective battery back or not. Finally they sent it back. So 3 months later and im stuck with my broken battery again.

Now I have a question for all you battery gurus. The far right cell has expanded and is defective. Can I just take that cell out and use the battery as a 4s battery??
Cheers

I'm sorry about your problems with HobbyKing; I hope that you're posting about your experience on their forums.

The answer to your question is YES! You will have a 14.8 volt battery instead of an 18.5 volt battery; it will power an MBA but not an MBP. There are some things to be mindful of:

Usually, the cells are arranged physically, 1 thru 5, in the same order as they are electrically, but there is no reason to assume that that is always true; make sure that your wiring is electrically correct, both for the power and the balancing harness.

Balancing harnesses do not have a standard connector or pinout; make sure that yours is wired for your charger or balancer. Check everything before you connect your charger or laptop.

If the battery that you use doesn’t have a protection circuit, you’ll need to get one of those in there, too.

Can you expand on the protection circuit? I have made one of these with a 4S lipo i.e. 14.8V wired up to a Magsafe using the bullet connectors described in the original post to power my MBA but I haven't connected it to my Mac yet because I'm concerned that connecting a raw battery directly to the MBA might damage it. That's something I can't afford. If there is some kind of protection circuit I can put between the 4S lipo and my MBA I'd feel a lot better about plugging it in. I've looked at the commercially available external batteries for Mac's and I'd be surprised if they didn't have some kind of protection circuit but I haven't been able to find out any info. Mark

Can you expand on the protection circuit? ... I'm concerned that connecting a raw battery directly to the MBA might damage it. ... If there is some kind of protection circuit I can put between the 4S lipo and my MBA I'd feel a lot better about plugging it in. ... Mark

That kind of protection is built into your Mac; if the input power is wrong, the Mac won't pass it through. The battery protection circuit that we are talking about here is to protect the battery; if a LiPo battery is fully discharged, it will be dead, as a doornail, forever. (This is part of the reason that so many devices use proprietary batteries; some have protection circuits in the battery, which makes them a little more expensive, and some have protection circuits in the device itself—it isn't a good idea to hack a battery for, let's say, one cell phone into another make/model, unless you understand what you're doing.)

The batteries that we're talking about using in this thread do not have built-in protection circuits, so it would be easy fully discharge them while using your laptop. The only risk in not using the protection circuit is to your rather expensive external battery.

Mayhem28 if you are still trying to get someone to make a battery I would be happy to. I have already sent a PM with all the details to everyone else that requested one but you have PM's blocked . If you are interested you can send me a PM and allow PM's to come in on your end and I will send you the details.

If anyone else is interested feel free to send me a PM and I will send you the details. All I would do is make the magsafe cord considering that is what requires soldering. It's about $26 for me to make and mail and I am only asking for a small tip for time and soldering equipment/connectors. I can help you pick out a battery and charger if needed.

Right on guys! I'm glad someone is able to start a little business making these for other people!

I'm the original thread starter, I did move to a balanced charger, I did not have any problems with a non balanced charger but thought my time might be running out so I bought the balanced charger. So far I have about 150 cycles of charges and discharges and no problems. It's great to keep in your laptop bag!

Hi Chip. I'm sure that it was mentioned earlier in the thread, but I will clarify it here for others; the only difference between balanced and unbalanced charging of multi-cell lipo batteries is the number of charging cycles during the battery's useful life—with balanced charging, a battery will usually two or more times longer. Balanced charging is much more efficient, and the cost difference between chargers will likely be made up with a single battery of the type and size that we are talking about here.

I haven't done it before, so I would really like to thank you for starting this thread—Thanks Chip; you da man.

I just buy them off ebay. You can buy just the cord with the magsafe and not the entire charger so it saves you a lot of money and time. They are around $15-$20, not sure if they are OEM but the one I bought works just fine and looks and feels just like the original cable.

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Quote:

Originally Posted by Chipg

Right on guys! I'm glad someone is able to start a little business making these for other people!

I'm the original thread starter, I did move to a balanced charger, I did not have any problems with a non balanced charger but thought my time might be running out so I bought the balanced charger. So far I have about 150 cycles of charges and discharges and no problems. It's great to keep in your laptop bag!

I need to get myself a balance charger, I bought the same charger you did in the beginning. It has been working fine but all of the cells in my battery are way off of voltage so I should probably buy a balance charger to get the most life out of my battery. I think I might also just have a defective battery, the first two cells in my battery deplete much faster than the last two. I'm hoping maybe a balance charger can help but if not I'll just try a new battery.

For those without balancing chargers, you can buy a LiPo cell balancer for less than $20 and stick that in the balance tap while charging. As a bonus, it'll read out each cell's voltage so you can monitor cell health.

Also, as far as protection circuits, just get a LiPo low voltage alarm and stop using it once the alarm activates. You'll be able to hear it clearly, it's meant to be heard from a flying model. These cost between $5-10 dollars.

I just buy them off ebay. You can buy just the cord with the magsafe and not the entire charger so it saves you a lot of money and time. They are around $15-$20, not sure if they are OEM but the one I bought works just fine and looks and feels just like the original cable.

Thanks Brandon; on a hunch, I did a search at aliexpress.com and found the MagSafe cables for as little as $7.92 each if you buy 20 or more, or $14.00 for one; both prices include shipping. I don't know if they're knock-off or are snuck out of the legit factory's back door, but if they work, it gives an advantage over cutting the cable off of your charger, or buying one or an Airline adapter. The wires are already prepped, too, making soldering easier.

My narrowed down search results are in the PDF below, and it also shows a MagSafe Car Charger $19.00 each, but you have to buy a lot of 30—that's $572.92—OUCH!!!

The pac stays really cool, it is a 15c pack which means it is made to put out voltage over a long time vs a battery like for a r/c helicopter that is made to disperse it's voltage fast. You will have ZERO probs with this battery and heat, it felt room temerature the whole time, perfect to leave in your brief case or backpack.

I'll get you guys some more details and info soon.

Lots of weird info about what the "C" rating means in this thread... here's the low-down...

RC planes/cars/helis put more strain on batteries than any other device, period. RC'ers are asking that these batteries go from full to dead flat in just a few minutes. Issue is, batteries from laptops and things can't handle it, they would puff or even explode.

The measure used to be "1C", which means you can draw from it its capacity rating. For example, a 4000mah pack can discharge at 4amps if it was only 1C. 1amp if it was a 1000mah pack. But, this would take one hour to discharge ("mAh" actually stands for "milliamps per hour").

So those in RC need more power! We want to get everything in minutes! So in fact, a 30C pack can be discharged at 30 times its capacity sustained... a 1000mah pack at 30C can deliver 30amps, a 4000mah pack at 30C is 120amps... at 30C current draw, the pack will last all of 2 minutes until it was dead flat. oh, but that 2 minutes is so much fun!

I have planes that suck down 2KW (yes, 2 kilowatts, 2000 watts) sustained for 4 minutes (and it's not a particularly big plane, but it is rather powerful) with two of these packs being discussed in this thread (two 4 cell LiPo, 25C 4500mah)... I could use one pack to run a MacBook Air for hours and hours (and yes, one 4000mah 14.8v 25C lipo can deliver 1000watts for a few minutes no problem... they're not kidding around).

...so, if getting batteries from an RC supplier, have no fear that it can supply the current for your computer, all you need worry about is capacity... as obviously that is what will determine how long the charge will last.

For computers, don't waste money on high discharge batteries... lower C ratings will be cheaper, and usually less weight (less to carry, woot!). 20C is as low as you'll get from most RC places (less than 20C to an RC'er isn't much fun)... but that is still 10 times more C rating than you need!

...just wanted to clear all that up

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Oh, and HobbyKing has the worst customer service in RC... AMain Hobbies, great retailer in the USA with great customer care.

...their ecopower packs are priced well, and some have hard cases (TSA prefers the hard cases because it looks less like something you've made yourself... ask me how I know ). The pack linked above is the hard case.

Hey guys, I have an aluminum case from an old external 3.5" HDD case. Was going to make one of these external batteries. My dimensions of the biggest size battery I can have is 5 and 3/4 x 4 x 1 inches. I have a MacBook Pro 13inch Aluminum, so figured i would need a 18.5v battery. Within reasonable price (under $65) what battery would you guys suggest? Thanks Andy

Hi Chip. I'm sure that it was mentioned earlier in the thread, but I will clarify it here for others; the only difference between balanced and unbalanced charging of multi-cell lipo batteries is the number of charging cycles during the battery's useful life—with balanced charging, a battery will usually two or more times longer. Balanced charging is much more efficient, and the cost difference between chargers will likely be made up with a single battery of the type and size that we are talking about here.

I haven't done it before, so I would really like to thank you for starting this thread—Thanks Chip; you da man.

Well there is another difference, balance chargers are a whole lot safer. If you have say a 4 cell battery, but one of the cells is either bad or simply weaker than the others and discharging faster, a non-balance charger will read the total voltage of the pack and keep charging until its fully charged, meaning the other 3 'good' cells are being overcharged to compensate for the weaker cell. A balance charger will bleed current from the good cells until the lower one catches up, or throw an error and stop charging all together if it detects a bad cell vs forcing current through it.

You certainly want the safety of balance chargers, ESPECIALLY if you're ordering batteries from hobbyking or any of the cheap Chinese knock offs. Batteries with bad cells aren't uncommon from there, I've received a few myself for my brushless RC vehicles and heli.

Also keep in mind, a 4-cell lipo battery has a NOMINAL (which is how they're measured) voltage of 14.8. Fully charged you're pushing 16.8 volts through your MBA.

A 5-cell which was recommended earlier for a MBP has a Nominal voltage of 18.5v and pushes 21v fully charged. I'm not so sure these are risks you want to take with your expensive laptops TBH.

Note 1: Nominal is the average voltage recorded from fully charged to fully discharge (for a lithium battery, a full discharge is typically 3 volts per cell)

Note 2: Nominal voltage for a lipo cell is 3.7v
Note 3: Full charge voltage for a lipo cell is 4.2v

Also, the discharge rate (C) isn't that important for this application. A higher C doesn't automatically discharge faster, it CAN discharge faster if the load requires it though, which is a good thing. Higher C rates can also maintain their voltage under load better than a battery with lower discharge (C) rating and run cooler doing it.